SYDNEY (Reuters) - Businesses in one of Australia's biggest states suffered an internet outage for more than six hours, thanks to a faulty network device, the country's No. 1 telecoms company, Telstra Corp Ltd, said on Friday.

The outage, Telstra's seventh this year, will pile pressure on the former state monopoly, beset by smaller rivals that are consolidating in a bid to chip away at its lengthy market dominance.

In an email, a Telstra spokesman said a "device in the network behaving in a way that wasn't expected" caused Thursday's outage that hit about a fifth of the firm's business customers in Victoria, a state of nearly 6 million people.

He gave no details of the malfunctioning device, but said the issue was resolved by late on Thursday.